A. Field of the Invention
This invention relates to the field of inspection, and more particularly to the field of ultrasonic inspection of metals.
B. Description of the Prior Art
Ultrasonic waves can be generated in metals by a Lorentz force mechanism and, in magnetic materials, by both the Lorentz force mechanism and a magnetostriction mechanism. In both mechanisms, a coil is centered between the poles of a magnet adjacent to the surface of the metal and an RF current is fed through the coil. In the Lorentz force mechanism, the current in the coil induces eddy currents in the metal which react against an applied static magnetic field to produce the forces that launch the ultrasonic waves. In the magnetostriction mechanism, variations in the magnetic field, caused by the RF current cause changes in length in the magnetic material to create ultrasonic waves. Such types of electromagnetic transducers are used to generate ultrasonic Lamb waves, Rayleigh waves, and angle shear and longitudinal waves; and typical prior art transducers are exemplified by the meander coil and magnet disclosed in U.S. Pat. Nos. 3,850,028 to R. B. Thompson, G. A. Alers (the present inventors), and M. A. Tennison.
Electromagnetic transducers are ideal for use in moving inspection stations because they can generate elastic waves (generally referred to as ultrasonic waves) in the material being tested, without requiring physical contact with the material. They are noncontact transducers. However, their application has been limited by problems such as: (1) low signal generating efficiency, (2) high noise levels when used in motion with respect to ferromagnetic materials, and (3) the requirement of a large static magnetic field in the region of the RF coils.
The efficiency of the transducer depends upon the flux density of the applied static magnetic field. The field is generally furnished by a dc electromagnet which is bulky and consumes considerable power. This is a particularly severe problem for totally self-contained inspection stations which travel through pipelines, as described in U.S. Patent Application, No. 731,199 filed Oct. 12, 1976, to R. B. Thompson, G. A. Alers (the present inventors) and M. A. Tennison. The size of the electromagnets limits the minimum size of the pipe which can be inspected, and the power consumed by the batteries feeding the electromagnets limits the intensity and duration of the magnetic field which can be obtained. Thus, a method or structure which increases the intensity of the magnetic field without increasing the size or power consumed by the electromagnet is necessary to improve the efficiency of the transducer.
In a magnetic material, the low signal-to-noise ratio of electromagnetic transducers is caused by the Barkhausen effect noise generated when the transducer moves across the surface of the material. This noise is sufficient to obscure reflections from small defects in the material, particularly at test speeds of 1 to 2 mph. Until the present invention, this noise could only be suppressed by application of strong magnetic fields to a region large compared to the transducer.